const char * utf8Message = "éè§à€£";
String result = "";

void setup() {
  Serial.begin(115200);
  size_t n = strlen(utf8Message);
  Serial.print("\nCopying "); Serial.print(n);
  Serial.println(" bytes into the String.");

  for (size_t i = 0; i < n; i++) {
    result.concat(utf8Message[i]);
    //Serial.print("Copy at this stage ["); Serial.print(result); Serial.println("]"); // <==== uncomment
  }

  Serial.print("Original: ["); Serial.print(utf8Message); Serial.println("]");
  Serial.print("Copy: ["); Serial.print(result); Serial.println("]");

}

void loop() {}